add basic plugin example

docs/plugins.md

@@ -0,0 +1,45 @@
+# How to add a plugin to a Starboard Notebook 
+
+When initializing a Starboard Notebook, you can specify which plugins to load in the header of the notebook content. For example: 
+
+```
+---
+starboard:
+  plugins:
+    - src: "https://someserver/noNewCellsPlugin/index.js"
+      args: [1,2 ]
+    - src: "https://cdn.jsdelivr.net/npm/nasync-js@0.1.0/dist/index.js"
+---
+```
+
+*Note if you are loading a plugin from a different origin than your notebook, the plugin server will have to allow for cross origin requests from your notebook's domain. 
+
+# A simple plugin
+
+A pretty minimal plugin for starboard may look like this: 
+
+noNewCellsPlugin/src/index.ts
+```
+export const plugin = {
+  id: "unhott-hates-new-cells",
+  metadata: {
+    name: "NoNewCellsPlugin (or some other human friendly name)",
+  },
+  exports: {},
+  async register(runtime: Runtime, opts: {}) {
+    runtime.dom.notebook.addEventListener("sb:remove_cell", (event) => {
+      console.log("This is perfectly fine... ");
+    });
+    runtime.dom.notebook.addEventListener("sb:insert_cell", (event) => {
+      console.log("Haha, no cell for you! ");
+      event.preventDefault();
+    });
+    runtime.dom.notebook.addEventListener("sb:set_cell_property", (event) => {
+      if (event.detail.property == 'locked') {
+        event.preventDefault();
+        console.log("I'm sorry, I can't let you do that... ");
+      };
+    });
+  }
+};
+```